iPhone 4S Wifi issues are extremely common. If you're Wifi switch is greyed out, or your Wifi status in settings is "Off" instead of "Not Connected" like the images below, then it would appear you are victim to a fairly widespread fault.
In most cases this is caused by some bad solder joints on a motherboard component. If that's the case it's easily repaired. You will be able to use your phone again to it's fullest potential.
Judging by how common this is, I believe it's a problem caused by the manufacturing process, but can be made worse by a drop or a jolt to the phone. Which in turn cracks an inherently weak solder joint.